Self-Selection of Feeding Substrates by Tenebrio molitor Larvae of Different Ages to Determine Optimal Macronutrient Intake and the Influence on Larval Growth and Protein Content

SIMPLE SUMMARY: Insects are widely discussed as being an alternative source of protein and a substitute for soy- and fishmeal. However, mass production is still expensive, so means are being sought to reduce production costs. One way is to keep the costs for insect diets low. One such possibility could be the use of food industry side streams or agricultural by-products to formulate an insect diet. This study tested different substrates as feed for mealworm larvae. These included by-products from grain processing such as husks or bran, pomace and other plant components. In many studies, feed is often tested individually, during which insect larvae are given a defined substrate as a single feed or in a two-component mixture, which they either eat or reject. It is often difficult to define the right nutrient composition for creating an insect diet. Therefore, the method of self-selection was used, in order to draw conclusions about the necessary nutrient requirements, especially depending on the age of the larvae. The results showed that the most suitable substrates for rearing mealworm larvae were mainly wheat bran and flour, maize hulls, oat bran and flakes, rice flour, lupine flour and potato flakes. Irrespective of their age, the larvae required a high content of carbohydrates and proteins and a low amount of fats and minerals. Indeed, protein and fat requirements grow with increasing age. These findings contribute towards formulating diet mixtures for Tenebrio molitor larvae of different ages. ABSTRACT: Nutrient self-selection was used to determine the optimal uptake of macronutrients by the yellow mealworm (Tenebrio molitor) larvae. The selection study consisted of four combinations of eight pelleted substrates from a total choice of 25, available to the larvae in a multiple-choice arena. In order to be able to determine the nutrient requirements as a function of the larvae age, six, eight and tenweekold larvae were used for the experiment. The larvae were free to choose between the different feeds for a period of two weeks. Rearing took place at 27 °C, 75% relative humidity and under dark conditions. The optimal ratios of macronutrients were 67.3 to 71.5% for carbohydrates, 19.9 to 22.8% for proteins and 8.6 to 10.0% for lipids to ensure the best results. Biomass growth, food intake and conversion were positively influenced to a significant extent by carbohydrate intake. The protein content, too, varied according to the macronutrient intake and substrate composition; a higher protein consumption increased the larval protein content. Wheat bran and flour, oat bran and flakes, maize hulls, lupine flour and potato flakes, in particular, were considered suitable substrates for the feeding and rearing of Tenebrio molitor larvae and highlighted that these larvae preferred a grain-based diet.